Made from renewable sources, bio-distillates as an alternative fuel for diesel engines is becoming increasingly important. In addition to meeting engine performance and emissions criteria/specifications, bio-distillates has to compete economically with petroleum-distillates and should not compete with food applications for the same triglycerides. Vegetable oils partially or fully refined and of edible-grade quality, are currently predominant feedstock for bio-diesel production. The prices of these oils are relatively high for fuel-grade commodities.
These considerations have led to efforts to identify less expensive materials that could serve as feedstock for bio-diesel production and to design chemical processes for their conversion. Thus, animal fats have been converted to bio-diesel [C. L. Peterson, D. L. Reece, B. L. Hammond, J. Thompson, S. M. Beck, “processing, characterization and performance of eight fuels from lipids”, Applied Engineering in Agriculture. Vol. 13(1), 71-79, 1997; F. Ma, L. D. Clements and M. A. Hanna, “The effect of catalyst, free fatty acids and water on transesterification of beef tallow”, Trans ASAE 41 (5) (1998), pp. 1261-1264], and substantial efforts have been devoted to the development of waste restaurant grease [M. Canakci and J. Van Gerpen, “Bio-destillates production from oils and fats with high free fatty acids”, Trans. ASAE 44 (2001), pp. 1429-1436; Y. Zhang, M. A. Dube, D. D. McLean and M. Kates, “Bio-destillates production from waste cooking oil. 1. Process design and technological assessment”, Bioresour. Technol. 89 (2003), pp. 1-16; W.-H. Wu, T. A. Foglia, W. N. Manner, R. O. Dunn, C. E. Goering and T. E. Briggs, J. Am. Oil Chem. Soc. 75 (1998) (9), p. 1173], largely the spent product of the deep fat frying of foods, as a bio-diesel feedstock.
The industrial chemistry of fats & oils is a mature technology, with decades of experience and continuous improvements over current practices. Natural fats & oils consist mainly of triglycerides and to some extent of free fatty acids (FFA). Many different types of triglycerides are produced in nature, either from vegetable as from animal origin. Fatty acids in fats & oils are found esterified to glycerol (triacylglycerol). The acyl-group is a long-chain (C12-C22) hydrocarbon with a carboxyl-group at the end that is generally esterified with glycerol. Fats & oils are characterized by the chemical composition and structure of its fatty acid moiety. The fatty acid moiety can be saturated or contain one or more double bonds. Bulk properties of fats & oils are often specified as “saponification number”, “Iodine Value”, “unsaponification number”. The “saponification number”, which is expressed as grams of fat saponified by one mole of potassium hydroxide, is an indication of the average molecular weight and hence chain length. The “Iodine value”, which is expressed as the weight percent of iodine consumed by the fat in a reaction with iodine monochloride, is an index of unsaturation.
Some typical sources of fats & oils and respective composition in fatty acids are given by way of example in Table 1.
TABLE 1SymbolCotton-CoconutCornPalmPeanutPalmLinseedRiceRape-OliveSaturatedCaproic 6:00.40.2Caprylic 8:07.33.3Capric10:06.63.5Lauric12:047.847.80.2Myristic14:00.918.116.30.11.10.40.02Palmitic16:024.78.910.98.511.644.16.019.83.910.5Margaric17:00.05Stearic18:02.32.71.82.43.14.42.51.91.92.6Arachidic20:00.10.11.50.20.50.90.60.4Behenic22:03.00.30.20.2Lignoceric24:01.00.20.1TOTAL28.091.922.782.020.3509.023.36.813.87UnsaturatedMyristoleic14:1 w-5Palmitoleic16:1 w-70.70.50.10.20.6Heptadecenoic 17:1 w-150.09Oleic18:1 w-917.66.424.215.438.037.519.042.364.176.9Linoleic18:2 w-653.31.658.02.441.01024.131.918.77.5Linolenic18:3 w-30.30.747.41.29.20.6Gadolenic20:1 w-91.00.50.51.00.3TOTAL72.08.177.318.079.7509176.793.286.13PolyunsaturatedRicinoleic18Rosin acids—% FFA0.5-0.61.0-3.51.70.10.82-1425-150.5-3.80.5-3.3Soy-Sun-LinolaLardButterfatTallowTallCastorJatrophaSaturatedCaproic2Caprylic2Capric3Lauric0.50.53.5Myristic0.10.21.5113Palmitic11.06.85.626262621.014.6Margaric0.50.5Stearic4.04.74.013.51122.511.07.4Arachidic0.30.420.5Behenic0.1LignocericTOTAL15.512.69.642.060.552.03.52.022.0UnsaturatedMyristoleic0.5Palmitoleic0.10.1422.50.8Heptadecenoic0.530.5Oleic23.418.615.9432643163.047.5Linoleic53.268.271.892.51.5204.228.7Linolenic7.80.52.00.540.31.0Gadolenic10.5TOTAL84.587.490.458.037.548.054.57.578.0Polyunsaturated24Ricinoleic89.5Rosin acids40% FFA0.3-1.60.1-1.50.30.55-20
Bio-distillates feedstock are classified based on their free fatty acid (FFA) content as follows [J. A. Kinast, “Production of bio-distillates from multiple feedstock and properties of bio-distillates and bio-distillates/-distillates blends”, NREL/SR-510-31460 report (2003)]:                Refined oils, such as soybean or refined canola oils (FFA<1.5%);        Low free fatty acid yellow greases and animal fats (FFA<4.0%);        High free fatty acid greases and animal fats (FFA>20.0%).        
Bio-diesel is currently produced by transesterification of triglyceride with methanol, producing methyl-ester and glycerol. This transesterification is catalyzed by homogeneous or heterogeneous basic catalyst. Typically homogeneous catalyst are alkali hydroxides or alkali alkoxides and typical heterogeneous catalyst are alkaline earth or zinc oxide materials, like zinc or magnesium-aluminate spinels. The presence of free fatty acids (FFA) in the raw triglycerides is a cumbersome for the production of bio-diesel as the FFA's react stoechiometrically with the basic catalyst producing alkali or alkaline soaps. This means that fats & oils that contain significant amounts of FFA's cannot be employed directly for bio-diesel production with this process. Several technical solutions have been proposed: (i) starting with an acid catalysed interesterification with additional glycerol to convert FFA's into glycerides prior to the basic transesterification; (ii) prior to the basic catalyzed transesterification the FFA's are removed by steam and/or vacuum distillation. The latter results in a net loss of feedstock for the production of bio-diesel. Eventually, the so produced FFA's can be converted by acid catalysis into esters in a separate process unit. FFA's can be present in triglycerides in different concentrations and can be present as such resulting from the extraction process or can be produced during storage as of the presence of trace amounts of lipase enzyme that catalyse the triglyceride hydrolysis or can be produced during processing, like thermal treatments during cooking.
There are other potential feedstock available at this time, namely trap and sewage grease and other very high free fatty acid greases who's FFA can exceed 50%.
The main sources of fats & oils are palm and palm kernels, soybeans, rapeseed, sunflower, coconut, corn, animal fats, milk fats.
Potentially new sources of triglycerides will become available in the near future, namely those extracted from Jatropha and those produced by microalgues. These microalgues can accumulate more then 30 wt % of lipids on dry basis and they can either be cultivated in open basin, using atmospheric CO2 or in closed photobioreactors. In the latter case, the required CO2 can originate from the use of fossil hydrocarbons that are captured and injected into the photobioreactor. Main sources of fossil CO2 are power stations, boilers used in refineries and steamcrackers furnaces used to bring hydrocarbon streams at high temperature or to supply heat of reactions in hydrocarbon transformations in refineries and steamcrackers. In particular steamcracking furnaces produce a lot of CO2. In order to enhance the CO2 concentration in flue gases of these furnaces, techniques like oxycombustion, chemical looping or absorption of CO2 can be employed. In oxycombustion, oxygen is extracted from air and this pure oxygen is used to burn hydrocarbon fuels as to obtain a stream only containing water and CO2, allowing concentrating easily the CO2 for storage or re-utilisation. In chemical looping, a solid material acts as oxygen-transfer agent from a re-oxidation zone where the reduced solid is re-oxidised with air into oxidised solid to a combustion zone, where the hydrocarbon fuel is burned with the oxidised solid and hence the effluent resulting from the combustion zone only contains water and CO2. Absorption of CO2 can be done with the help of a lean solvent that has a high preferential to absorb CO2 under pressure and typically at low temperature and will release the CO2 when depressurised and/or heated. Rectisol® and Selexol® are commercial available technologies to remove and concentrate CO2. Other sources of CO2 are the byproduct from carbohydrates fermentation into ethanol or other alcohols and the removal of excess CO2 from synthesis gas made from biomass or coal gasification.
US 2007/0175795 reports the contacting of a hydrocarbon and a triglyceride to form a mixture and contacting the mixture with a hydrotreating catalyst in a fixed bed reactor under conditions sufficient to produce a reaction product containing diesel boiling range hydrocarbons. The example demonstrates that the hydrotreatment of such mixture increases the cloud point and pour point of the resulting hydrocarbon mixture.
US 2004/0230085 reports a process for producing a hydrocarbon component of biological origin, characterized in that the process comprises at least two steps, the first one of which is a hydrodeoxygenation step and the second one is an isomerisation step. The resulting products have low solidification points and high cetane number and can be used as diesel or as solvent.
US 2007/0135669 reports the manufacture of branched saturated hydrocarbons, characterized in that a feedstock comprising unsaturated fatty acids or fatty acids esters with C1-C5 alcohols, or mixture thereof, is subjected to a skeletal isomerisation step followed by a deoxygenation step. The results demonstrate that very good cloud points can be obtained. US 2007/0039240 reports on a process for cracking tallow into diesel fuel comprising: thermally cracking the tallow in a cracking vessel at a temperature of 260-371° C., at ambient pressure and in the absence of a catalyst to yield in part cracked hydrocarbons.
U.S. Pat. No. 4,554,397 reports on a process for manufacturing olefins, comprising contacting a carboxylic acid or a carboxylic ester with a catalyst at a temperature of 200-400° C., wherein the catalyst simultaneously contains nickel and at least one metal from the group consisting of tin, germanium and lead.
It has been discovered a process to make bio-naphtha and bio-diesel in an integrated biorefinery from all kinds of natural triglycerides or fatty acids. In said process crude fats & oils are refined, either physically or chemically, to remove all non-triglyceride and non-fatty acid components. The refined oils are next fractionated in both liquid and solid fractions. This process aims at separating a starting material into a low melting fraction, the liquid fraction, consisting of triglycerides, having double bonds in the acyl-moieties and a high melting fraction the solid fraction, consisting of substantially saturated acyl-moieties. This process allows optimising the use of the different molecules constituting the natural fats & oils. Bio-destillates require specific cold-flow properties that requires double bonds in the acyl-moiety. On the other hand, the quality of a steamcracker feedstock is better when the hydrocarbon is saturated and linear.
The liquid fraction, potentially mixed with some limited solid fraction, is transesterified with a C1 to C5 monofunctional alcohol to produce alkyl fatty esters, called also bio-diesel, and glycerol. The amount of solid fraction should be so that the final cold-flow properties are according to the local market specifications.
The solid fraction, potentially mixed with some liquid fraction, can be converted to produce bio-naphtha and optionally bio-propane. The solid fraction can be directly hydrodeoxygenated or can also be hydrolyzed to give fatty acids, potentially mixed with those produced during refining. Then fatty acids can be hydrodeoxygenated or decarboxylated to bio-naphtha. The solid fraction can also be saponified to produce glycerol and soap that can subsequently be decarboxylated.
As several sources of fats & oils are not suitable to be converted in ester-type bio-diesel because they contain too much saturated acyl-moieties that result in high pour-points and hence improper cold-flow properties, the present invention solves this problem by an appropriate separation of the starting complex mixtures, allowing an optimal usage of fats & oils for making bio-diesel and bio-naphtha.
The use of a biofeed is a possible solution in the search of alternative raw material for the naphthacracker. Nevertheless, using this type of feed can lead to corrosion problems and excessive fouling because of oxygenates forming from the oxygen atoms in the biofeed. Also existing steamcrackers are not designed to remove high amounts of carbonoxides that would result from the steamcracking of these biofeedstock. According to the present invention, such a problem can be solved by hydrodeoxygenating/decarboxylating (or decarbonylating) this biofeed before its injection into the steam cracker. Thanks to this hydrodeoxygenation/decarboxylation (or decarbonylation), the negative effect due to the production of CO and CO2 and traces of low molecular weight oxygenates (aldehydes and acids) in the steam cracker is reduced.
Another advantage is of course the production of bio-monomers in the steam cracker.